Diamonds in Wisconsin? You bet!
Most diamonds form within the earth’s mantle at depths of 100 to 200 km. They are brought to the surface by the action of deep-seated magma which crystallizes to form pipe to dike shaped bodies of the igneous rock called lamproites. Since diamond is physically and chemically resistant, it survives weathering and can be transported long distances from its source by rivers or glaciers. When deposited it occurs with the denser minerals such as magnetite, garnet and gold. A number of diamonds, however, have been found through the years in glacial drift and river sediments in Wisconsin. Recently, a diamondiferous lamproite was discovered in the subsurface in Kenosha, Wisconsin. This suggests a local source for at least some of the drift diamonds, although others could have been transported longer distances, from Michigan or Canada.
DANE COUNTY: A grayish green 3.34 (3.83?) carat diamond was found in the drift of the Johnstown moraine 2.5 miles southwest of Oregon on the Charles Devine farm in 1893. It is described as “a rhombic dodecahedron, deeply pitted with circular elongated reniform markings” (Kunz,1894, Hobbs, 1894, 1901; Cannon and Mudrey, 1981).
LANGLADE COUNTY: Panning in 1987 turned up several tiny diamonds in stream sediments near Antigo. One such examined by the author was a clear sharp octahedron approximately 1 mm in diameter.
KENOSHA COUNTY: A subsurface diamond-bearing lamprophyre diatreme has been discovered in Kenosha. The body has an area of about 50 acres. Drill core samples have small but “sharp-edged octahedra or macles, some showing intricate growth patterns and trigons”. Associated minerals include pyrope (both low Cr and high Cr), Cr-diopside, picroilmentite, Cr-spinel, serpentine, calcite and micas. Traces of millerite, celestine, barite and apatite are also reported. (Carlson and Adams, 1997).
OZAUKEE COUNTY: A white 6.57 carat diamond was found in 1881 near the Milwaukee River, about 4 km. north of Saukville. The diamond is described as rounded, almost spherical with dodecahedral faces (Olson, 1958; Vierthaler,1958; Cannon and Mudrey, 1981).
MANITOWOC COUNTY: Several uncut diamonds were found in 1913 in the cabin of a desceased hermit near the town of Collins. Cannon and Mufrey (1981) suggest that the diamonds were found in local gravels.
PIERCE COUNTY: At least 10 small yellowish diamonds were found from1890 to 1892 during gold placering operations in near the junction of Plum Creek and Rock Elm Creek near Rock Elm. A second find in 1906 has been alleged to be bogus (Cannon and Mudrey, 1981; Cordua, 1987).
RACINE COUNTY: In1903, an irregular rounded 2.11 carat diamond was found in Burlington (Hobbs, 1901; Cannon and Mudrey, 1981). Previously a 2.06 carat white twin was reported found there in 1897 (Eckert, 1980).
WASHINGTON COUNTY: The Theresa diamond, weighing 21.5 carats, was found in 1888 on or near the Green Lake Moraine near Kohlsville. The diamond was half cream yellow and half colorless. It has since been cut into 10 separate stones by the owner (Cannon and Mudrey, 1981).
WAUKESHA COUNTY: A16.25 carat diamond of a “warm sunny color” was reportedly found by drilling in 1876 near Eagle. The diamond has a dodecahedral form with triangular and circular markings on the faces. The locality was later salted with diamonds and other stones, a situation finally debunked by George Kunz. The Eagle diamond was in the American Museum of Natural History until stolen in 1964 ( Kunz, 1894; Olson, 1953,Cannon and Mudrey, 1981).
Source: https://home.wgnhs.wisc.edu/minerals/diamond/
Check out more minerals of Wisconsin, including fluorescent minerals, at the Wisconsin Geological and Natural History Survey website for indepth mineral locations and maps.
